Xenomai在不同域中的优先级问题

First example:
---------------------
Suppose that at a rescheduling we have the following thread in the ready
queues
- High Priority -> Linux thread H
- Medium Priority -> Xenomai thread M in PRIMARY domain
- Low Priority -> Xenomai thread L in primary domain

Then, the Xenomai scheduler will select M, and M will be dispatched.

Second example:
--------------------------
- High Priority -> Linux thread H
- Medium Priority -> Xenomai thread M in SECONDARY domain
- Low Priority -> Xenomai thread L in primary domain

Then, the Xenomai scheduler will select M. but M is in secondary mode,
which means the Linux kernel will be scheduled. The Linux scheduler
knows H (it's a linux thread), M (it's in secondary mode), and maybe
about L (if it has been created by UVMs). The Linux scheduler will
choose H, and H will be scheduled.

比如例一，在Xenomai域中的低优先级的线程M(低于Linux域的线程H)会被调度。

例二，如果线程M进入Linux域后，这里比较关键，不是线程Ｌ被调度，而是在Linux域中的高优先级线程Ｈ被调度。

如果在Xenomai域中有优先级高于线程Ｍ的线程Ｚ，那么Ｚ则会被调度。

参考：

http://www.xenomai.org/pipermail/xenomai/2005-December/001881.html

http://www.xenomai.org/pipermail/xenomai/2005-December/001882.html